// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// https://developers.google.com/protocol-buffers/
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package com.google.protobuf;

/**
 * Abstract base interface for protocol-buffer-based RPC services. Services themselves are abstract
 * classes (implemented either by servers or as stubs), but they subclass this base interface. The
 * methods of this interface can be used to call the methods of the service without knowing its
 * exact type at compile time (analogous to the Message interface).
 *
 * <p>Starting with version 2.3.0, RPC implementations should not try to build on this, but should
 * instead provide code generator plugins which generate code specific to the particular RPC
 * implementation. This way the generated code can be more appropriate for the implementation in use
 * and can avoid unnecessary layers of indirection.
 *
 * @author kenton@google.com Kenton Varda
 */
public interface Service {
    /**
     * Get the {@code ServiceDescriptor} describing this service and its methods.
     */
    Descriptors.ServiceDescriptor getDescriptorForType();

    /**
     * Call a method of the service specified by MethodDescriptor. This is normally implemented as a
     * simple {@code switch()} that calls the standard definitions of the service's methods.
     *
     * <p>Preconditions:
     *
     * <ul>
     *   <li>{@code method.getService() == getDescriptorForType()}
     *   <li>{@code request} is of the exact same class as the object returned by {@code
     *       getRequestPrototype(method)}.
     *   <li>{@code controller} is of the correct type for the RPC implementation being used by this
     *       Service. For stubs, the "correct type" depends on the RpcChannel which the stub is using.
     *       Server-side Service implementations are expected to accept whatever type of {@code
     *       RpcController} the server-side RPC implementation uses.
     * </ul>
     *
     * <p>Postconditions:
     *
     * <ul>
     *   <li>{@code done} will be called when the method is complete. This may be before {@code
     *       callMethod()} returns or it may be at some point in the future.
     *   <li>The parameter to {@code done} is the response. It must be of the exact same type as would
     *       be returned by {@code getResponsePrototype(method)}.
     *   <li>If the RPC failed, the parameter to {@code done} will be {@code null}. Further details
     *       about the failure can be found by querying {@code controller}.
     * </ul>
     */
    void callMethod(
            Descriptors.MethodDescriptor method,
            RpcController controller,
            Message request,
            RpcCallback<Message> done);

    /**
     * {@code callMethod()} requires that the request passed in is of a particular subclass of {@code
     * Message}. {@code getRequestPrototype()} gets the default instances of this type for a given
     * method. You can then call {@code Message.newBuilderForType()} on this instance to construct a
     * builder to build an object which you can then pass to {@code callMethod()}.
     *
     * <p>Example:
     *
     * <pre>
     *   MethodDescriptor method =
     *     service.getDescriptorForType().findMethodByName("Foo");
     *   Message request =
     *     stub.getRequestPrototype(method).newBuilderForType()
     *         .mergeFrom(input).build();
     *   service.callMethod(method, request, callback);
     * </pre>
     */
    Message getRequestPrototype(Descriptors.MethodDescriptor method);

    /**
     * Like {@code getRequestPrototype()}, but gets a prototype of the response message. {@code
     * getResponsePrototype()} is generally not needed because the {@code Service} implementation
     * constructs the response message itself, but it may be useful in some cases to know ahead of
     * time what type of object will be returned.
     */
    Message getResponsePrototype(Descriptors.MethodDescriptor method);
}
